The present invention relates to an inverter apparatus, and more particularly to an inverter apparatus in which after disconnecting an a.c. power source, residual electric charge of a smoothing capacitor is discharged by turning on a semiconductor switching element at a desired time or after a lapse of an allowable duration of momentary power stoppage so that electrical shock by residual voltage may be prevented.
In general, an inverter apparatus is provided with a d.c. smoothing capacitor having a large electrostatic capacity, and a voltage remains on this smoothing capacitor for a long time after the input voltage is cut off unless special measures of discharge are taken.
Since a dangerous electrical shock may be encountered in touching the inside of an inverter apparatus at maintenance and the like, electric charge of the d.c. smoothing capacitor is discharged in a conventional inverter apparatus by devices as disclosed in JP-A-59-172984 (1984), JP-A-60-9396 (1985), JP-A-61-191299 (1986), JP-A-62-114492 (1987), and JP-A-64-8894 (1989). FIG. 8 shows an example of these prior arts, in which a contact b of an electromagnetic contactor 84 is closed when the a.c. input voltage is cut off, thereby absorb as heat energy the residual voltage of a smoothing capacitor CB through a resistor RB.
Any of the above-described prior art, however, is not arranged to immediately discharge the electric charge of the d.c. smoothing capacitor at any desired time, and hence it is impossible to immediately discharge the electric charge of the d.c. smoothing capacitor for the purpose of preventing a danger when some serious situation happens, for instance. Further, it is not necessarily recommendable to discharge the electric charge of the d.c. smoothing capacitor every time the power source is turned OFF, because it creates a problem that charge and discharge are repeated frequently in case the start-stop frequency is high, thus reducing the life of the d.c. smoothing capacitor, a contactor, a rush current preventing resistor and the like.
Further, in the above-described prior art, since the electric charge of the d.c. smoothing capacitor is discharged independently of the allowable duration of a momentary power stoppage, it creates a problem that there is a possibility of a large rush of current at a time of power restarting if the electric charge of the d.c. smoothing capacitor has been discharged, thereby destroying elements of a power rectifier.